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lief
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2楼2009-03-27 12:32:22
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Abstract In a multicomponent reaction (MCR), one can create multiple new bonds in a single reaction from readily available starting materials; thus, MCRs are resource-and timeeffective and therefore economically favorable processes in diversity generation. In contrast, there are MCRs where a multifunctional building block is introduced instead of an additional diversity-holding component, and these can be derivatized using very diverse reactions post-synthetically leading to novel chemotypes. The synthetic applications of Meldrum*s acid are focusing primarily on reactions where it is applied as an alternative for acyclic malonic esters. However, its highly acidic character broadened its applications and made it a very useful reagent for MCRs or more precisely in tandem or domino reactions. There are numerous examples reported for the use of the alkylidene conjugates of Meldrum*s acid as dienophiles in hetero-Diels –Alder reactions, as well as Michael acceptors. In most cases spontaneous or concomitant post-synthetic derivatization increased its synthetic utility. This minireview gives a non-exhaustive insight into MCRs involving Meldrum*s acid, describing various applications in combinatorial and diversityoriented synthesis |
3楼2009-03-27 12:39:00
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1 Introduction Multicomponent Reactions (MCRs) have been a versatile tool for synthetic chemists in the preparation of structurally diverse compounds. MCRs comprise reactions with more than two reactants and the newly formed product contains atoms of each precursor [1]. In contrast, the probability that three or more molecules collide in the right direction and at the appropriate energy level is very low, most of the known MCRs could be considered more precisely as domino or tandem reactions. MCRs are resourceand time-effective, and therefore economically favorable processes; thus, a vast number of diverse compounds can be obtained in a parallel synthesis [2]. In recent years, there has been a growing interest in MCRs in the chemical and pharmaceutical industries, as MCRs not only lower production costs due to their high convergence and atom efficiency, but also reduces the environmental burden, which is the major principle of green chemistry. The enormous synthetic possibilities that MCRs offer can be further increased by post-synthesis transformations, which can be furnished by concomitant reaction of a suitably functionalized or protected MCR product. These modifications can be either spontaneous reactions with the medium, intramolecular rearrangements, or can take place upon treatment with additional reagents. Meldrum*s acid (1) described first by A. N. Meldrum [3] is a white crystalline solid that can be easily prepared by the condensation of malonic acid and acetone in acetic anhydride in the presence of a catalytic amount of concentrated sulfuric acid [4]. Meldrum*s acid shows several unique features. It has an unusually high acidity [5]; the pKa of Meldrum*s acid in DMSO is 7.325, but those of dimedone and dimethyl malonate, corresponding to the cyclic diketone and acyclic ester analogues, are 15.87 and 11.16, respectively. Furthermore, it is susceptible to electrophilic attack at C5 and nucleophilic attack at C4 and C6. Additionally, its unique ring-opening reactions make it a tremendously attractive and useful building block. The synthetic applications of Meldrum*s acid are focusing primarily on reactions where it is applied as an alternative for acyclic malonic esters, but there are numerous examples reported for the use of the alkylidene conjugates of Meldrum*s acid as dienophiles in hetero-Diels –Alder reactions. In most cases spontaneous or concomitant post-synthetic derivatization increases its synthetic utility. The MCRs involving Meldrum*s acid generally retain the unique ring-captured malonic acid moiety, which can be released by loss of acetone, when reacting with nucleophiles. This reaction is frequently accompanied with partial decarboxylation. In this way various diversity elements can be built into the diverse MCR products. Frequently, nucleophiles in appropriate proximity could intramolecularly attack the cyclic acetonide fragment, leading to unique ring systems, which can be used in Diversity-Oriented Synthesis (DOS) [6]. Based on the above findings, the MCRs involving Meldrum *s acid belong to those classes where it participates as a multifunctional building block in the reaction instead of one diversity-holding component, so the MCR product can be further derivatized in a large variety of reactions leading to diverse skeletons or chemotypes. Corresponding to the initial step and the primary intermediate formed involving Meldrum*s acid, the MCRs can be classified into various subgroups. In most cases, a reactive alkylidene Meldrum*s acid intermediate (a Knoevenagel adduct) participates in various secondary reactions. This two-step feature is reflected, in many cases, in the name of MCRs involving Meldrum*s acid [domino Knoevenagel –Diels –Alder, domino Wittig –Knoevenagel – Diels –Alder, modified Hantzsch reaction, Yonemitsu reaction (domino Knoevenagel –Michael reaction), etc.]. In most of the above cases Meldrum*s acid, condensed first with carbonyl moieties, could ultimately lead to a substituted propionic acid extension of the molecules. Domino Knoevenagel-isonitrile-cycloaddition represents a unique subclass, since the major product retains both carboxylic groups of the masked malonic acid moiety. Some MCRs cannot be clearly classified where, for example, Meldrum*s acid reacts with unsaturated carbonyl compounds in a Knoevenagel condensation and ring closure is followed by condensation. In other cases Meldrum *s acid acts as a Michael donor with its highly acidic methylene moiety in an aldol-type reaction. In the present minireview we follow this classification providing a general description and examples to each subclass. |
4楼2009-03-27 12:39:44